1. Field of the Invention
The present invention relates to a method of diagnosing the fatigue life of structural framework such as buildings, bridges, and other constructions whose main structures are made of steel, and a member of steelwork having a life diagnostic function.
2. Description of the Related Art
A member of steelwork constituting a building is susceptible to fatigue damage more or less by vibrations caused by the wind or earthquake after it has been built up. The fatigue and damage progress in bridges due to repeated vibrations caused by vehicles passing on them. To prevent destruction resulted from the fatigue damage, the following conventional methods have been practiced: a method of calculating a degree of strain in a structure in advance and estimating the life of the structure; and a method mainly using visual inspection such as a color check if it is assumed that cracking has occurred due to the fatigue and damage. These conventional methods cannot accurately diagnose an actual degree of fatigue damage to determine whether to repair. There has recently been proposed a vibration-damping device having a vibration-damping function, in which low-yield steel is incorporated in structual component with a form of a wall or brace. No technique for accurately diagnosing the degree of fatigue damage and determining whether to repair is developed for the above device.
An attempt has been made to nondestructively inspect the material and stress of a target measurement object by using the fact that the magnetic properties of the material depend on the microstructures, such as the crystal grain size and a precipitate or the like, and strain.
The following examples are disclosed as a method and/or apparatus for nondestructively inspecting the degree of degradation of a material: a method of inspecting the degree of degradation by calculating the ferrite generation amount of a high-temperature gas furnace member from its magnetic susceptibility (Japanese Patent Laid-Open No. 59-135362); an apparatus for diagnosing the degree of material degradation of a steel pipe by eddy current (Japanese Patent Laid-Open No. 60-257354); a method of diagnosing the degree of material degradation of a turbine rotor from its ECT value and hardness to predict the destruction lifetime (Japanese Patent Laid-Open No. 61-172059); an apparatus for detecting the degree of degradation of low-alloy steel from its magnetic permeability (Japanese Utility Model Laid-Open No. 61-161659); a method of measuring changes in magnetic domain wall movement characteristics of a ferromagnetic body over time to inspect the degree of brittlement of the material by comparing the measured changes with a master-curve obtained in advance (Japanese Patent Laid-Open No. 1-269049); and a method of quickly measuring the degree of degradation of a metal material in a radiation environment by using a plurality of magnetic sensors (Japanese Patent Laid-Open No. 4-125463).
The above conventional techniques, however, detect material degradation caused by changes in microstructures such as phase transformation when a target measurement object is set in a high-temperature state or exposed to radiation. These conventional techniques are not directed to detection of fatigue degradation caused by the stress or strain, which is the object of the present invention. In addition, the detection means used in the conventional techniques are the eddy current and magnetic permeability and do not use Barkhausen noise.
The following methods are proposed as a method using Barkhausen noise: a method of estimating the degree of fatigue degradation of mild steel (L. P. Karjalainen et al., IEEE Trans. Mag. MAG-16, 514 (1980)); and a method of estimating the toughness of tool steel (Nakai et al., Iron and Steel, 75,833 (1989)). The result by L. P. Karjalainen in IEEE Trans. Mag. MAG-16, 514 (1980) exhibits a change in Barkhausen noise in a fatigue test in which a magnetic head is brought into tight contact with a thin mild steel piece. More specifically, this reference exhibits an abrupt change in Barkhausen noise prior to the fracture of the test piece. No description, however, is made for Barkhausen noise measurement conditions under which the degree of fatigue can be accurately detected in an actual structure and the life of the actual structure can be diagnosed, provided that the magnetic head cannot be brought into direct contact with a measurement location due to, e.g., an uneven surface or the presence of a covering material.
As described above, the fatigue damage by stress and strain in structural steelwork are conventionally diagnosed by mainly visual inspection. Such a diagnosis is performed upon removing a wall or covering material in a building or the like, resulting in high cost. Visual inspection is also mainly performed for bridges. The visual inspection never detects cracking until it has developed and grown up to some extent. A location where an operator cannot access due to the structural limitation cannot be subjected to the fatigue diagnosis. Neither a method of diagnosing the fatigue life of an actual structure nor a member of steelwork capable of practicing the method is available.